1. Field of the Invention
Embodiments of the present invention are directed to (oxy)nitride-based phosphor compositions that may be excited by radiation ranging from the UV to the blue region of the electromagnetic spectrum, and under such excitation, emit in the green region. The field of the invention includes considerations of the structure of such phosphors, the methods of their synthesis, and related white light-emitting devices.
2. Description of the Related Art
Certain nitride and (oxy)nitride-based phosphors are known in the art. In general, they fall into one of two different types or classes of compounds; both are nitrides, but only one contains oxygen. For convenience, the nomenclature of these nitride/oxynitride-based phosphors may be abbreviated to (oxy)nitride-based, and the terms “nitride/oxynitride” and “(oxy)nitride” will be considered to be equivalent in this disclosure.
One such nitride relevant to the present disclosure is based on a host having the formula SrAlSi4N7. This compound is known to emit in the orange-to-red region of the electromagnetic spectrum when activated by the divalent rare earth Eu2+. It has been discussed at length in the literature, in one such paper by Hecht et al., as well one by Ruan et al. The relevance of this discussion will become apparent in a later section, where modifications to the SrAlSi4N7 host crystal structure are discussed in the context of the present embodiments.
In one embodiment of the present invention, the green-emitting, (oxy)nitride-based phosphors are derived from α-Si3N4, rather than from β-Si3N4, is the case with the conventionally known, green-emitting β-SiAlON. The present green-emitting (oxy)nitride based phosphors are configured to emit radiation having a wavelength ranging from about 500 to about 550 nm when excited by blue or UV light (e.g., radiation ranging from about 200 nm to about 470 nm), and differ from the β-SiAlONs in structure. Two papers from the literature have described the prior art phosphor SrAlSi4N7. These papers will be briefly reviewed.
Hecht et al.
A description of one such nitride-based phosphor was provided by C. Hecht et al. in an article titled “SrAlSi4N7:Eu2+—A nitridoalumosilicate phosphor for warm shite light (pc)LEDs with edge-sharing tetrahedral,” Chem. Mater. 2009, 1, 1595-1601. This phosphor emits in the red, and thus its use allows a white light illumination product to be “warm,” with high color rendering. Structure-wise, the compound features infinite chains of edge-sharing AlN4 tetrahedra.”
Hecht et al. teach that SrAlSi4N7:Eu2+ may be synthesized from the starting materials Sr metal, α-Si3N4, AlN, and Eu metal. The raw powders were sintered in a nitrogen atmosphere at temperatures up to 1630° C. to produce a crystalline substance in the orthorhombic crystal system with the space group Pna21 (No. 33). The lattice parameters are a=11.7 Å, b=21.39 Å, and c=4.97 Å. The Eu2+ doped compound SrAlSi4N7 exhibits the emission 4f65d1→4f7 in the orange-red region of the spectrum (for 2% Eu doping and 450 nm excitation, λmax=632 nm). The network structure of this compound, was characterized by corner-sharing SiN4 tetrahedra incorporating infinite chains of edge-sharing AlN4 tetrahedra, whose direction runs parallel to the [001] direction (i.e., the c-axis).
The manner in which the tetrahedra are linked has other implications. As taught by Hecht et al. in Chem. Mater. 2009, 1, 1595-1601, the tetrahedra in SrAlSi4N7 are linked not only by “linked common corners,” where the vertices of two adjacent tetrahedra are shared; there are also configurations where two adjacent tetrahedra are joined by a common edge. Contained within the SrAlSi4N7 structure are infinite chains of trans-edge-sharing tetrahedra that run along the [001] direction of the crystal (in other words, parallel to the c-axis). Hecht et al. suggest that the cations occupying each of the edge-shared tetrahedra in the infinite chains are trivalent aluminum cations, Al3+. The trans-edge-shared AlN4 tetrahedra in the infinite chains are cross-linked via corner-shared SiN4 tetrahedra. This leads to channels along the c direction where the modifying cations, in this case the divalent alkaline earth elements such as Sr, are inserted. The channels host two different types of alkaline earth (Sr) positions that are coordinated by irregular polyhedral consisting of either six or eight nitrogen atoms.
The diagram for space group is Pna21 (No. 33) in International Tables For Crystallography, Volume A, Space-Group Symmetry, edited by T. Hahn, second, revised edition (International Union of Crystallography, Boston, 1987), pp. 224-225. It indicates that there is only one special site per unit cell; each of the 10 different Si/Al sites in the unit cell of SrAlSi4N7 resides on a Wyckoff position 4a. Hecht et al. teach that chemical and structural analyses “suggest that two out of these 10 positions should be exclusively occupied by Al [.]. Unlike Si4+, aluminum (Al3+) seems to be predestined [to be] situated on the tetrahedral centers of the infinite chains [making up the] edge-sharing tetrahedral [AlN4].”
Ruan et al.
Ruan et al. teach, in an article titled “Nitrogen gas pressure synthesis and photoluminescent properties of orange-red SrAlSi4N7:Eu2+ phosphors for white light-emitting diodes,” J. Am. Ceram. Soc., 94 [2] 536-542 (2011), that the enhanced mechanical properties of nitride-based phosphors are explained, in large part, by the stiff frameworks provided by a highly condensed network of corner and edge-shared SiN4 and AlN4 tetrahedra. Such compounds exhibit enhanced optical properties as well: rare earth doped (oxy)nitride phosphors have excitation bands covered by the emission of commercially available InGaN-based LED chips “because of the strong nephelauxetic effect and large crystal field splitting caused by the [higher] coordination of the nitrogen [anions relative to the 0 anions of an oxide-based phosphor].”
Ruan et al. reviewed the work of Hecht et al. (discussed above), and state in view of the latter's disclosure: “It is obvious that there are two different Sr lattice sites in SrAlSi4N7.” It appears that they do not sit on the corners of an orthorhombic unit cell (the crystal system of the recently disclosed orange-red SrAlSi4N7:Eu2+ phosphors). More precisely, there are two Sr (modifier cation) interstitial sites per unit cell, and they differ in size and coordination number. The Ruan et al. article is relevant to the present disclosure in that it discusses the difficulties encountered with the synthesis of the title compounds.
Ruan et al. make a critical observation about the synthesis of the SrAlSi4N7: “the reported synthesis of SrAlSi4N7:Eu2+ based on radiofrequency furnace is usually baffled by the byproducts of microcrystalline Sr2Si5N8.” While not wishing to characterize Ruan et al.'s statement, it is certainly understood as the present inventors have experienced similar “difficulties” with the appearance of undesired phases, particularly Sr2Si5N8. While not wishing to be bound by any particular theory, it is believed that the kinetics and/or thermodynamics of the synthesis are such that a Sr2Si5N8 phase has, in certain situations, a lower free energy of synthesis than does the desired SrAlSi4N7, and with this in mind, steps may be taken to modify traditional solid state reaction conditions so as to favor the synthesis of the desired SrAlSi4N7 compound.
Ruan et al. discuss the structure of a modification. Their synthesis of crystalline SrAlSi4N7:Eu2+ via gas pressure sintering, using starting materials that included Sr3N2, α-Si3N4, AlN, and EuN. The mixtures were fired at 1750° C. for 2 hours under a 0.48 MPa nitrogen atmosphere in a gas pressure sintering furnace. Subsequently, the samples were mixed with additive AlN “with contents equal to the first time” and refired for 6 hours under the same conditions.
Ruan et al. reported that the pure SrAlSi4N7 phase was difficult to achieve with the starting raw materials mixed stoichiometrically. See J. Am. Ceram. Soc., 94 [2] (2011), p. 537. They reported that “additional AlN may be helpful to solve the problem.” X-ray diffraction (XRD) data of samples taken after the first firing, where starting material ratios were calculated based on the desired stoichiometric composition (e.g., SrAlSi4N4), and after the second firing, where excess AlN has been added. Ruan et al. showed that their data shows that the primary phase of the product after the first firing was Sr2Si5N8, with only a small amount of the desired SrAlSi4N7 compound being produced. After the second firing, where excess AlN had been added to the mix, the main phase was the desired SrAlSi4N7 phase, and interestingly, almost no AlN or Sr2Si5N8 could be recognized. The authors state that the concentration of AlN is critical to the formation of the SrAlSi4N7 phase, at least under their reaction conditions, and the broader point may be made that reaction mechanism(s) leading to the formation of the SrAlSi4N7 phase are not fully understood.
These two research papers reveal that the synthesis and properties of the compound SrAlSi4N7 are known. They establish a base compound that can be modified.